1. Field of the Invention
The present invention relates to a method of manufacturing high permeability magnetic cores usable for noise filters, zero-phase current transformers, magnetic heads, saturable reactor, small-sized spike noise reducing inductor, etc. which require a higher permeability material.
2. Description of the Prior Art
A higher performance has recently been required for magnetic parts in proportion to the higher performance of electronic devices. Accordingly, excellent magnetic characteristics have been required for magnetic materials used for these magnetic parts. A high permeability material is effective to various magnetic parts of current sensors such as zero-phase current transformes, noise filters, etc.
The magnetic material used for a noise filter will be described hereinbelow by way of example.
A switching power supply is widely used as a stable power supply for computer peripheral devices or general communication systems. In the switching power supply, noise voltage is often generated in the device being superimposed upon the power supply voltage when the supply voltage is applied from an external power supply line. In addition, there exists another problem such that higher noises (with the switching frequency as the fundamental frequency) or noises on the order of MHz generated from a load such as logical circuits incorporated in a computer will be generated and inputted to the device.
In order to reduce these conductive noises, a common mode choke coil for reducing noise between a device and the ground as shown in FIG. 1 is often used as a noise filter. In FIG. 1, a pair of windings 3a, 3b are wound around a magnetic core 2 in such a way that two magnetic fluxes each generated by an alternating current passed through the winding can be cancelled out each other. Further, capacitors 4a, 4b and 4c are connected between these windings 3a and 3b, and a junction point between the two 4b and 4c is gounded.
When the filter as described above is connected to the power supply line, the ratio of noise output voltage to noise input voltage is closely related to the permeability of the magnetic core of the filter, in other words, the higher the permeability is, the smaller will be the noise output voltage. Further, excellent frequency characteristics of the permeability are also required because the filter should function effectively even at a high frequency range of 1 MHz or more in addition to a low frequency range.
Conventionally, ferrite has been used as material for manufacturing the magnetic core of the common mode choke coil. However, recently, since there exists a strong demand for noise reduction even at a relatively low frequency range of 10 to 450 kHz, a problem arises in that ferrite cannot sufficiently reduce low frequency noise because the permeability of ferrite is relatively low at low frequencies. Therefore, magnetic core materials provided with a higher permeability at low frequency range and stable frequency characteristics have strongly been needed.
On the other hand, recently, the availableness of amorphous alloys for the common mode choke coil has been studied because of its higher permeability. However, the amorphous alloys are not yet satisfactory with respect to the noise level reduction at lower frequencies, and further the researches for higher permeability amorphous alloys may reach a limit for the future if only the alloy composition will be studied.
In the magnetic core for noise filters, a high permeability is required over a wide frequency range. Further, a higher permeability contributes to the miniaturization, the higher precision, and the higher sensitivity of apparatus including magnetic cores. Therefore, there exists a strong demand for higher permeability material available for magnetic apparatuses.